Surface Acoustic Wave (SAW) devices, on account of their small size, low cost and ease of mass production, have been widely accepted in many applications such as cellular phones, wireless LANs, and cellular base stations. Typically, a SAW device includes transducers and/or reflectors disposed on a surface of a piezoelectric substrate. FIG. 1a illustrating a typical SAW resonator filter comprising transducers and reflectors. The transducers, composed of metalized interdigitated electrodes, are used for the generation and detection of surface acoustic waves. The electrode widths and spacing determine the frequency characteristics of the SAW filter. Lithium Tantalate and Lithium Niobate piezoelectric substrates are commonly used for the manufacturing of SAW devices. The strong coupling characteristics of these piezoelectric substrates provide a very desirable substrate for which low insertion loss and large fractional bandwidth SAW filters can be designed. However, both substrates exhibit high temperature drift by way of example, a 64° YX cut Lithium Niobate would typically exhibit about 70 ppm/° C. shift while a 46° cut Lithium Tantalate (LT) would shift about 50 ppm/° C. Thus, for a PCS hand phone that operates at a center frequency of about 1.9 GHz and over a temperature range of 100° C. would result in a frequency shift of about 14 MHz for Lithium Niobate and 9.5 MHz for Lithium Tantalate. This high frequency drift would preclude some of the SAW applications that may require a very steeply shaped frequency response. A proven technique for limiting the temperature drift of a SAW filter is to bond a thin layer of Lithium Tantalate or Lithium Niobate to a low coefficient thermal expansion carrier substrate like Si or Glass as disclosed by Taguchi et al in U.S. Pat. No. 5,998,907. The combined structure of the SAW metallized pattern on the surface of a piezoelectric substrate mounted on a carrier substrate (also designated as surrogated substrate) is referred to as a “composite SAW die.” FIG. 1b illustrates a composite SAW die in which the piezoelectric wafer is bonded directly on carrier substrate while FIG. 1c depicts a composite die structure with the piezoelectric substrate bonded to the carrier through a catalytic bonding layer. Si has a coefficient of thermal expansion (CTE) of about 2.6 ppm/° C. and that of Lithium Tantalate is about 16 ppm/° C. The relatively low CTE silicon constrains the high CTE Lithium Tantalate during thermal excursions thus limiting the temperature drift of the filter response. The composite bonded die structures typically are mounted in packages using conventional chip and wire methods as illustrated in FIG. 2a. Bonded wires form the electrical interconnects from the SAW device pattern to the signal pads of the package. Signal pads are connected either to the input transducer, output transducer or ground pad of the SAW device. Chip and wire SAW packages have a disadvantage in that they require a certain amount of space clearance for bonding wire to the package signal pads. In quest of miniaturization, the SAW die can be mounted in a flip chip configuration as illustrated in FIG. 2b. Here, the die is mounted in a face down manner connecting the SAW pattern structure directly to the signal pads of the package, thereby eliminating the need for wire bonding. Typically, flip chip SAW devices exhibit a much lower height and smaller size profile than the chip and wire SAW devices and thus are generally preferred.
FIGS. 3a and 3b illustrate, by way of example, temperature characteristics of an 1880 MHz SAW composite filter for a chip and wire ceramic package and a flip chip type package respectively. The three curves represent the frequency responses approximately at −20° C., 25° C. and 70° C. respectively. FIG. 1 identifies an average temperature drift of the conventional (no temperature compensation) and composite (temperature compensated) SAW filters for both package types. It is clear from the table of FIG. 1 that the composite SAW device for both the chip and wire and flip chip are significantly improved compared to the conventional SAW filters. Typically, for conventional non-temperature compensated SAW filter, it is expected that the temperature drift for the flip chip SAW filter behaves better than that of the chip and wire device as is shown in the Table. Thus, it is expected that the flip chip composite SAW filter exhibit better temperature characteristics than the composite chip and wire device.
However, an unexpected result was discovered. The flip chip composite SAW filter exhibits a higher temperature drift than that of the composite chip and wire device. In particular, the temperature drift at the lower 8 dB frequency point shows approximately 8 ppm/° C. worse and the 20 dB frequency point shows approximately about 3 ppm/° C. worse. By way of example, for mobile telephone applications, a 3 ppm/° C. would translate into an additional frequency drift of approximately greater than 0.5 MHz which is very significant. It was discovered that a key disadvantage of a flip chip package attaching these bonded composite structures to ceramic packages is that the intermediate CTE of the ceramic (7 ppm/° C.) reduces the amount of temperature compensation created by the silicon. Basically, the bonded die structure is rigidly attached to the ceramic package during flip-chip assembly. Since the ceramic expands at a greater rate than the silicon, the temperature compensation created by the thermal constraint of the silicon on the Lithium Tantalate is lower than expected. This is in direct contrast to chip and wire applications in which a low stress adhesive is typically used to attach the bonded die structures to the high temperature co-fired ceramic (HTCC) ceramic package. The adhesive effectively decouples the silicon from the HTCC and allows the silicon to properly constrain the planar expansion of the Lithium Tantalate during thermal excursions.
Thus, it is highly desirable to obtain a composite SAW die device that will exhibit low height profile and small size that can maintain good temperature characteristics of the temperature compensation of the composite SAW die. The composite SAW device should preferably be bond wire free. It is desirable to provide the teaching of techniques for packaging a composite SAW die bonded directly to a substrate that exhibits a low coefficient of thermal expansion. It is also desirable that the composite SAW device be capped by a substrate whose coefficient is very similar to that of the carrier substrate of the composite SAW die.